This invention relates generally to systems for removing adherent material, such as paint or other coverings, from surfaces, and more particularly to nonchemical surface cleaning systems employing mechanical blast.
For various types of structures and equipment, it is often necessary or desirable to remove the layer or layers of coatings which have been applied to surface areas. Numerous techniques exist for removing paint, sealants, lacquers and other adherent materials from virtually any type of surface. Surface cleaning or stripping methods range from mechanical abrasion to the use of strong chemicals, and involve varying degrees of time, effort and expense. For any given type of coating, the character and function of the substrate material from which a coating is to be removed ususally dictates the stripping method, at least in industrial settings. Hard, durable surfaces, such as heavy steel plating, can be cleaned or stripped by relatively fast abrasive methods, such as sand blasting. More delicate surfaces may require careful chemical removal to prevent damage or destruction of the substrate.
A certain class of materials, generally called composites, present special problems which have heretofore required the use of expensive and hazardous chemical treatments to remove surface coatings. Composites are usually made of a matrix material, such as plastic or epoxy, which often contains fibers such as glass strands, graphite, kevlar or the like for reinforcement. Layers of the material are laminated together or pressed onto a honeycomb base to form structural material. Composites are strong and light and are increasingly used in aircraft and other manufactured products where weight savings are important. Because composites usually have surfaces which are softer than metals, removal of paint or other coatings from composites must be done carefully to avoid excessive abrasion or chemical damage.
The greatest costs in both time and money associated with stripping and cleaning composites are probably encountered in the maintainance of modern aircraft, which incorporate large areas of exterior surface formed of composites. Airlines and the military spend large amounts chemically stripping paint and other coatings from aircraft, in preparation for repainting. The weight savings from stripping generally justifies the enormous expenditure in man-hours to strip an aircraft using chemicals and sanding.
Recent developments have indicated the effectiveness of a new stripping technique, similar to sand blasting, which is quicker and safer than chemical stripping. The system uses a granular media consisting of numerous particles of a plastic material accelerated to high speed and directed against the surface to be cleaned. The media particles can be of various sizes, depending on the application, and can be accelerated to produce a continuous media flow using conventional sand blasting equipment. This system has been shown to be highly effective in removing paint and other coatings from harder surfaces, such as metal, and also for deburring and other finishing processes and the like. It is far safer than chemical stripping, presents no hazardous waste disposal problems, and greatly reduces the man-hours and expense of surface cleaning. Blast cleaning with plastic media has been shown to be effective on the metal parts of aircraft, but was not previously considered suitable for stripping composites. Due to the relative softness of composites, as compared to metal, plastic media blast cleaning by prior art methods tended to score, abrade or otherwise damage composite surfaces to an unacceptable degree. Until the development of a blast cleaning method which solves such problems, aircraft and other surfaces made of composites have had to be cleaned and stripped by laborious and expensive prior art techniques.
It would be advantageous to have a less hazardous and more economical method of cleaning and stripping composite surfaces. The use of blast cleaning techniques for cleaning composites would be especially desirable since it would greatly reduce the cost and time for such cleaning. Any blast cleaning method used on composites must, however, not result in damage to the composite surface.